I can only imagine how difficult it must be to let your horses go. Even when you know its best for them it doesn't mean you don't still miss them.
2 of my 3 horses were very loved and missed by their previous owners. They were only sold because the people were ready to move on to barrel racing and another one could no longer ride.
I feel very blessed that they allowed my family to become the new owners, these horses were meant to be for us and I still send them photos and updates of our adventures on trails and camping once a month.
i think the fact they get to see them, hear about them and know they are well loved and taken care of give them comfort.
Maybe you could ask whoever gets your horses if they would do the same.
Don't take life so seriously. After all no one gets out alive.